The present invention relates generally to motor drives for electrical motors and in particular to a motor drive useful in safety systems.
Motor controllers provide sophisticated control of electrical motors by changing the frequency and amplitude of the power received by those motors. The control provided by a motor drive may be simple open-loop control of the motor, for example, ramping its speed up during start or may include more complex feedback control of motor speed and position using signals from an external tachometer or position encoder. One or more motor drives may be connected to an industrial controller programmed to coordinate the actions of multiple motors in an industrial process.
Safety systems are systems intended to ensure the safety of humans working in the area of potentially dangerous machinery in an industrial process. Safety systems accept inputs from emergency stop buttons, rope pull switches, safety mats, key switches, light curtains and the like, and stop the operation of equipment based on a command or the presence of a human in a dangerous area. Typically such equipment is powered by electric motors, and safety systems may disable those motors to stop the equipment.
Traditionally safety systems have been implemented by a set of circuits wholly separate from an industrial control system that may be used to control the industrial process with which the safety system is associated. Such safety systems may use specially designed “safety relays” which require a redundant signal to actuate relay contacts and which provide feedback signals that can reveal relay failures such as welded or stuck contacts.
In order to simplify wiring and hardware when motor drives are part of a safety system, it is known to incorporate safety relay circuitry directly into the motor drive. In this way, redundant safety signals may be provided directly to the motor drive (and safety feedback signals returned) allowing safe shutdown of the motor without the need for external contactors and their associated wiring.
The safety wiring connected to the motor drive includes five or seven conductors: three for the two redundant safety signals and a combined return which drive separate relay coils, and two or four more for the feedback signals and returns for two separate contacts associated with each relay coil. Often multiple motor drives are stacked together in a small area and adding this additional wiring to each motor drive can be cumbersome. When multiple motor drives are used, the number of wires from the control device to each motor drive can exceed the limit of wires that can fit on the terminals of the control device requiring the use of additional terminals and additional wiring. Inevitably, connecting the large number of wires needed for safety control increases the possibility of miswiring and increases installation costs.